This invention relates to electrical filters and, in particular, to dielectric filters that provide increased attenuation proximate to the desired passband.
Ceramic block filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, the resonators are formed by cylindrical passages, called holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator holes.
One of the two opposing sides containing holes is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions are complex and difficult to predict. These
These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands.
Although such RF signal filters have received wide-spread commercial acceptance since the 1970s, efforts at improvement on this basic design continued.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of filter technology to provide sufficient frequency selectivity and band isolation.
Coupled with the higher frequencies and crowded channels are the consumer market trends towards ever smaller wireless communication devices (e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators or allow greater insertion loss in order to provide improved signal rejection.
Therefore, the need continues for improved RF filters which can offer selectivity and other performance improvements, without increases in size or cost of manufacturing. This invention overcomes the size-to-selectivity compromise by providing a ceramic block RF filter having adaptable selectivity with a robust, relatively low cost control mechanism and relatively low insertion loss.
The present invention is a preferred duplexer filter that is a monolith (also referred to as a monoblock) of a dielectric ceramic that defines a plurality of resonators. The preferred filter has at least three input/output (I/O) pads. One of the pads is coupled to an antenna, another is connected to a transmission circuit and the last pad is connected to a receive circuit. The filter is comprised of two sections: a transmission section and a receive section. The transmission and receive sections include resonators disposed on respective sides of the antenna pad.
The filter of the invention also includes a first alternative signal path adjacent the ends of the transmission resonators. A second alternative signal path is disposed adjacent to the ends of the resonators. Each alternative signal path couples adjacent and non-adjacent resonators. A further feature of the filter of the present invention includes a shunt zero resonator for the transmission section. To the contrary, the present invention allows the elimination of a shunt zero resonator for the received section of the filter.
Specified more generally, a preferred RF signal filter according to the present invention includes a block of dielectric material having an input electrode and an output electrode spaced apart along the length of the block. The block defines an array of through-hole resonators extending between the input electrode and the output electrode. A resonator by-pass electrode extends from a position adjacent a first resonator of the array to a position adjacent a second resonator of the array. The first and second resonators are separated by at least one resonator of the array such that the by-pass electrode provides a parallel signal pathway between the first and second resonators.
There are other advantages and features of this invention which will be more readily apparent from the following detailed description of the preferred embodiment of the invention, the drawings, and the appended claims.